Venue: CGFB
Language of defense: English
Team: Motoneurons and synaptic partners (MotoPSyn), INCIA
Thesis supervisor: Pascal Branchereau
Title
Prenatal dysfunctions of chloride-related inhibition in lumbar motoneurons of the SOD1G93A ALS mouse model
Abstract
Amyotrophic lateral sclerosis (ALS) is a fatal and adult-onset neurodegenerative disease characterized by progressive degeneration of motoneurons (MNs) with complex multifactorial aetiology. Most ALS studies have focused on symptomatic stages based on the hypothesis that ALS pathogenesis occurs when the disease becomes symptomatic. However, growing evidence indicates that ALS pathogenesis might start long before symptom onset. My PhD thesis work was based on the hypothesis that ALS – familial and sporadic – stems from deficits taking place during early development. With the aim of identifying early changes underpinning ALS neurodegeneration, the first part of my thesis analysed the GABAergic/glycinergic inhibitory postsynaptic currents (IPSCs) to embryonic (E) E17.5 MNs located in the ventro-lateral motor column from SOD1G93A (SOD) mice, in parallel with the analyse of chloride homeostasis. Our results showed that IPSCs are less frequent in SOD animals in accordance with a reduction of synaptic VIAAT-positive terminals in the close proximity of MN somata. SOD MNs exhibited an ECI 10 mV more depolarized than wild type (WT) MNs. This deficit in GABA/glycine inhibition was due to a reduction of the neuronal chloride transporter KCC2. SOD spontaneous IPSCs and evoked GABAAR-currents exhibited a slower decay correlated to elevated [Cl-]i. Using computer modelling approach, we revealed that the slower relaxation of synaptic inhibitory events acts as a compensatory mechanism to strengthen or increase the efficacy of GABA/glycine inhibition when ECI is more depolarized. Interestingly, simulations revealed an excitatory effect of low frequency (<50Hz) depolarizing GABA/glycine post-synaptic potentials (dGPSPs) in SOD-like MNs but not in WT-like littermates. At high frequency, dGPSPs switched to inhibitory effect resulting from the summation of the shunting components. The second part of my PhD thesis focussed on the effect of electrically evoked-dGPSPs, at different frequencies (7.5 to 100 Hz), on real lumbar E17.5 MNs in which a depolarized ECI (below spike threshold) was imposed. The aim was to examine whether the excitatory effect could be linked to morphological changes previously described in E17.5 SOD MNs. Results showed that some MNs were excited by low-frequency dGPSPs and inhibited by high-frequency dGPSPs (Dual MNs) and others were inhibited at all frequencies (Inhibited MNs). Dual effect was more often detected in SOD MNs. WT MNs were classified into two clusters according to their input resistance (Rin), Dual MNs being specific to high Rin and Inhibited MNs to low Rin. Morphometric data pointed out a reduced dendritic tree in high Rin WT Dual MNs and a large dendritic tree in low Rin Inhibited MNs. This was not the case in SOD MNs that were excited or inhibited whatever their morphology and Rin. In agreement with simulation showing that a less density of inhibitory current on MNs soma favors excitatory dGPSPs, we found less synaptic VIAAT terminals on the soma and proximal dendrites of SOD MNs, compared to littermate WT MNs, as well as a lower frequency of spontaneous dGPSPs. Altogether, my thesis data emphasize a prenatal defect in the Cl- homeostasis and GABA/glycine innervation in the SOD1G93A ALS MNs. Before birth, a dominant population of MNs with low Rin emerges in WT animals. These MNs that are inhibited by dGPSPs could represent future ALS-vulnerable fast MNs (putative FF). Interestingly, those MNs are not inhibited in SOD animals. The inhibitory dysfunction could be attributed to two distinct factors: morphology and perisomatic inhibitory synapse density. Of these two factors, the latter plays a major role in controlling the capability of GABAergic/glycinergic neurons for shaping spinal motor output.
Keywords: chloride homeostasis | synaptic integration | KCC2 | inhibition| spinal motoneurons| amyotrophic lateral sclerosis | SOD1G93A mouse model | simulation | patch-clamp | Prenatal stage | GABAergic/glycinergic synaptic transmission | GABA/glycine.
Publications
Hongmei Zhu, Urvashi Dalvi, William Cazenave, Daniel Cattaert and Pascal Branchereau. Excitatory action of low frequency depolarizing GABA/glycine synaptic events is favored in prenatal spinal SOD1G93A motoneurons. Submitted to Neurobiology of Disease.
Pascal Branchereau, Elodie Martin, Anne-Emilie Allain, William Cazenave, Laura Supiot, Fara Hodeib, Amandine Laupénie, Urvashi Dalvi, Hongmei Zhu, Daniel Cattaert. Relaxation of synaptic inhibitory events as a compensatory mechanism in fetal SOD spinal motor networks. eLife, 2019, 8:e51402.
Jury
Mme Muriel Darnaudéry
Pr, NutriNeurO, Université de Bordeaux Présidente
M. Daniel Zytnicki
DR, SPPIN, Université Paris Cité Rapporteur
M. Christophe Porcher
Pr, INMED, Université Aix-Marseille Rapporteur
Mme Sabine Lévi
DR, INSERM UMRS-1270, Paris Examinateur
M. Pascal Legendre
DR, INSERM U1130-CNRS UMR 8246-Sorbonne Université, Paris Examinateur